There have been more strict requirements for silver halide emulsions for photographic use, which has increased the demands for the high level photographic characteristics such as, for example, high speed, excellent graininess, high sharpness, low fog, wider exposure latitude range and so on.
The above mentioned requirements have been satisfied by well-known silver bromoiodide grain emulsions having a high silver iodide content in the inner part of the grains and a specific core-shell structure in the grains thereof. It is well known in the photographic art that light absorbing increases in the order of silver chloride, silver bromide and silver iodide, but development activity correspondingly decreases in the same order. By using the above described core-shell silver bromoiodide emulsions, a good balance between light sensitivity and development activity has been obtained.
Examples of core-shell silver bromoiodide emulsion are described in many patent and literature references. For example, U.S. Pat. No. 4,668,614 and U.S. Pat. No. 4,728,602 describe a monodispersed core-shell silver bromoiodide emulsion having a core part comprising a silver iodide content of 10 to 45 mol % and a shell part comprising a silver iodide content lower than 5 mol %, with an average silver iodide content higher than 7 mol %. When examined by X-ray diffractometry, two peaks are evidentiated. The first one corresponding to the high iodide core part, the second one to the low iodide shell part. According to the claimed invention it is preferred to have a ratio between the diffraction intensity of the two peaks in the range of from 1/10 to 3/1, more preferably 1/3 to 3/1.
Similarly, European application EP 299,719 discloses a core-shell silver halide emulsion having a core comprising not less than 10 mol % of silver iodide, at least one shell consisting of silver bromide or silver bromoiodide, the outermost of which has a silver iodide content not higher than 5 mol %, and an average silver iodide content of not less than 10 mol %.
EP 309,119 discloses a core-shell silver halide emulsion having at least three silver bromide or silver bromoiodide phases of different composition. According to a preferred embodiment of the claimed emulsion, the innermost phase has a silver iodide content of at least 10 mol %, the outermost phase has a silver iodide content of not more than 6mol %, and the intermediate phase has a silver iodide content difference with the outermost or innermost phase of at least 3mol %. When examined by X-ray diffraction, the claimed emulsion shows three or more diffraction peaks, each corresponding to a phase containing a different percentage of iodide.
EP 202,784 describes a core-shell type silver halide emulsion having an inner core essentially consisting of silver bromide or silver bromoiodide and a plurality of shells. The outermost shell has a silver iodide content ranging from 0 to 10 mol %, the innermost shell has a silver iodide content at least 6 mol % higher than that of the outermost shell, and an intermediate shell has a silver iodide content is at least 3 mol % lower than that of the innermost shell and at least 3 mol % higher than that of the outermost shell.
Finally, U.S. Pat. No. 4,477,564 describes a multiphase bromoiodide emulsion having an average silver iodide content higher than 12%.